The aunt of 18 year-old Yanira Serrano enters Serrano's home in Half Moon Bay on Wednesday morning. Serrano, armed with a knife, was shot and killed Tuesday night by a San Mateo County sheriff's deputy.

Photo: Kurtis Alexander, The Chronicle

The aunt of 18 year-old Yanira Serrano enters Serrano's home in...

Image 2 of 2

The aunt of 18 year-old Yanira Serrano enters Serrano's home in Half Moon Bay on Wednesday morning. Serrano, armed with a knife, was shot and killed Tuesday night by a San Mateo County sheriff's deputy.

(08-18) 12:45 PDT HALF MOON BAY -- A San Mateo County sheriff's deputy will not face criminal charges for fatally shooting an 18-year-old woman who suffered from schizophrenia and was armed with a knife near Half Moon Bay, prosecutors said Monday.

Deputy Menh Trieu, a nine-year department veteran, shot Yanira Serrano-Garcia on June 3 after she confronted the deputy with a 10 1/2-inch kitchen knife 30 seconds after he arrived at the Moonridge housing complex on Miramontes Point Road, authorities said.

The woman had the knife raised above her head as she ran toward the deputy "in what can be described as a primal, incoherent and garbled tone," District Attorney Steve Wagstaffe wrote in a report.

Trieu repeatedly told Serrano-Garcia to stop while retreating 150 feet backward to get away from her, but Serrano-Garcia continued running toward him "with the knife poised to strike," prompting Trieu to fire a single shot into her chest, the prosecutor said.

"Deputy Trieu exercised his police powers in an appropriately decisive manner against a person who was imminently trying to stab him with a large kitchen knife," Wagstaffe wrote.

"Confronted by this armed suspect, Deputy Trieu was entitled to use deadly force to protect himself, as well as any others in the immediate vicinity, including arriving officers who might have been endangered by Ms. Serrano-Garcia's knife attack."

Serrano-Garcia's brother had called 911, saying his sister was acting violently inside the family home. Family members have said Serrano-Garcia had special needs and had stopped taking her medication.

Wagstaffe said Trieu was aware from the beginning that Serrano-Garcia was mentally ill but that "his life was no less in danger."

In a statement, Sheriff Greg Munks said he extended his sympathies to Serrano-Garcia's family over a "tragic occurrence" and said Trieu, now working in the agency's corrections division, "is still dealing with the gravity of the incident."